Various techniques for reducing the effect of random noise on the quality of a television image have been proposed. Filtering or integrating circuits have been used, for example, but reduce contrast and picture definition. Systems have been proposed which compare information from frame to frame to evaluate motion and to change the integration constant of the filter to limit the filtering effect when picture content is changing more rapidly. In U.S. Pat. No. 3,009,016, for example, a noise reduction system is disclosed which attempts selective processing to suppress noise by taking advantage of the fact that noise is not as objectionable in picture regions of fine detail as compared to noise in regions of little or no contrasting detail, such as regions of uniform brightness. As described in the patent, a matrix of picture samples surrounding the sample being tested were used to develop average values in both the x and y directions. The differences between each picture sample and these averages were compared with a reference signal and if the differences exceeded the reference, the sample was transmitted without alteration. If no differences are exceeded, an average of all the samples in the matrix is substituted for the video signal.
The present invention, like the above-identified patent, takes advantage of the fact that noise in areas of high detail is less objectionable to the viewer than noise in areas of low detail. However, the present invention utilizes information derived from the region of the picture appearing above and to the left of the picture element being processed so that information on picture elements yet to be scanned in the same line or subsequent lines of the raster need not be available either from previous frames or by introducing delays of one or more lines.